This invention relates to the field of semiconducting devices.
The design requirements of missile defense and space surveillance systems have created a need for detection and imaging systems capable of operating in the long wavelength infrared (LWIR) range. These systems impose critical performance limitations on such parameters as resolution, field-of-view, operating temperature, responsivity, detectivity, ease of calibration, and radiation hardness. In particular, the need to improve resolution and field-of-view has established the desirability of high density, large area arrays of LWIR detectors. With the large amounts of data which will be generated from these arrays, on-focal-plane signal processing will be necessary to enable scene discrimination as well as to reduce data link requirements. Silicon-based devices would appear to be well suited to meet these needs, since the large scale integration (LSI) techniques which have been extensively developed for that technology can be combined with extrinsic silicon detector technology to fabricate monolithic and hybrid focal planes.
The aforementioned systems, however, must be capable of operating in the presence and aftermath of nuclear events. Under these conditions, nuclear radiation induced ionization pulses (spikes) at the detector output can introduce an additional noise component which will reduce the ability of such a system to detect faint targets and will add to the burden of reliably interpreting the focal plane output. Thus the utility of a detection and imaging system can be enhanced by reducing the sensitivity of the system to nuclear radiation.
Although such radiation induced noise can be diminished by decreasing the detector thickness, the thickness of a conventional extrinsic silicon detector cannot be reduced sufficiently without sacrificing detector performance, leading to unacceptable dark current levels, increased optical cross talk, and degradation at low background levels by response anomalies. Blocked-impurity-band detectors, which are described in U.S. patent application Ser. No. 199,881, filed Oct. 23, 1980, provide an effective solution to the problem of operating in the nuclear radiation environment. The structure of these detectors exhibits an inherent superiority in terms of nuclear radiation hardness and reduced optical crosstalk between adjacent detectors in closely spaced arrays. Furthermore, it has also been demonstrated that blocked-impurity-band detectors are free from the types of irregular behavior, such as memory effects, pulse shape variation, nonlinear responsivity, nuclear radiation induced responsivity variations, etc., which are observed in conventional extrinsic silicon photoconductive detectors. The resulting superior frequency response and stability of calibration are substantial assets in optimizing the performance of a sensor system.
In addition to its use in a detector, however, it would also be advantageous for the impurity band conduction concept to be available in other devices. Active circuit elements incorporating this concept, for example, might find many applications. Such circuit elements could be used to fabricate multiplexers, preamplifiers, and many other active devices.